Do I paint under the decking before laying?

Is it recomended to paint under the decking before laying it, I'm using single sided reeded Merbau

Is there a product i can use?

No just do the top after its laid.. I use a product from intergrain called ultradeck. 3 coats and it looks great... its easier than the usual 3 step process of other intergrain products.. Im sure others will have their favourites to use but ultradeck works for me.

I usually want my decks to last.

I use decking oil (or preservative) or get the owner to do it as many times as possible, all four sides, three of them will never see preservative for the rest of their lives.

I do the same with timber joists, oil the tops as much as possible.
Doubly so for the end grains.

Cheers
Bill.